There will be explained an example of a hydraulic shovel according to related art, with reference to FIG. 4.
The hydraulic shovel includes a crawler type lower traveling body 1; an upper slewing body 2 disposed thereon so as to be capable of being slewed around axis X perpendicular to a ground surface; a cabin C disposed on the upper slewing body 2 as a control room; and a front attachment AT attached to the upper slewing body 2. The front attachment AT is used to perform for example an excavation, including a boom 3 so as to be able to be raised and lowered; an arm 4 mounted on a distal end of the boom 3; a bucket 5 mounted on a distal end of the arm 4; and respective hydraulic actuators for operating them, namely, a boom cylinder 6, an arm cylinder 7, and a bucket cylinder 8. The lower traveling body 1 includes crawler type left and right traveling units 9, both of which are driven by traveling motors (not shown) that are traveling hydraulic motors.
The hydraulic shovel also includes a slewing motor (not shown) that is a hydraulic actuator for slewing the upper slewing body 2; a hydraulic pump for supply hydraulic oil to the hydraulic actuators; and a plurality of control valves disposed between the hydraulic pump and respective hydraulic actuators, each of the hydraulic actuators is driven by the hydraulic oil supplied through the control valves. Each of the control valves is for example a pilot controlled selector valve and is operated by a remote control valve.
In the hydraulic excavator, when a real speed of the hydraulic actuator becomes greater than the speed corresponding to a flow rate from the hydraulic pump, for example, when the slewing motor or the traveling motor is braked, there will be a possibility of cavitation. As a conventional means for preventing cavitation from occurring, provided is a back-pressure compensation valve in a return pipe line interconnecting each of the control valves and a tank. The back-pressure compensation valve generates back pressure to prevent cavitation from occurring.
FIG. 5 schematically shows a conventional back-pressure system. The back-pressure system includes a hydraulic pump 50; a hydraulic actuator exemplified as a hydraulic motor shown in FIG. 5; a control valve 54; a return pipe line 56 connecting the control valve 54 to a tank T; and a back-pressure compensation valve 58 disposed in the return pipe line 56. The back-pressure compensation valve 58 is generally constituted of a fixed type having a fixed setting pressure, adapted to generate back pressure corresponding to the setting pressure.
On the other hand, Patent Literature 1 discloses a technique with use of a variable back-pressure compensation valve having a variable setting pressure. According to this technique, the cavitation protection function of the back-pressure compensation valve is secured by setting the setting pressure of the variable back-pressure compensation valve to a high pressure when a machine is slewed or travels, while a power loss is allowed to be decreased by setting the setting pressure of the variable back-pressure compensation valve to a low pressure when each cylinder in the front attachment is operated.
Each of the above techniques with use of the fixed back-pressure compensation valve and variable back-pressure compensation valve involves a problem of increase in power loss. The fixed back-pressure compensation valve, whose setting pressure is fixed to a constant high pressure from a point of view of prevention of cavitation, can generate unnecessarily high back pressure in a situation of no possibility of cavitation to thus involve an excessive power loss. Also the related art with use of the variable back-pressure compensation valve can involve an unnecessary power loss because the setting pressure thereof is always set to a high pressure regardless of the operation amount when the machine is being traveling. Specifically, in actual, the cavitation can be caused under the condition of low traveling operation amount such as a case where the machine is slowly traveled on a downhill; meanwhile, when the machine is being traveled at a high speed, for example, by full operation of the traveling remote control valve, hydraulic oil is introduced into the traveling motors at a great flow rate to thus increase the circuit pressure loss and to thereby generate back pressure enough to lower the possibility of cavitation. Therefore, the related art where the setting pressure of the back-pressure compensation valve is unconditionally set to a high pressure when the machine is being traveling also involves the problem of generating unnecessary backpressure, when the machine is being traveled at a high speed, to thus increase power loss.